1. Field of the Invention
The present invention relates to a vehicular headlamp having a plurality of light source units and more particularly to a vehicular headlamp capable of controlling various light distribution patterns in response to changes in the vehicle's driving status and achieving lower power consumption.
2. Description of the Related Art
Headlamps equipped with light source units that have light-emitting diodes (LEDs), laser diodes (LDs), or similar semiconductor light-emitting elements as their light sources have been proposed for use in vehicles. In such headlamps, a plurality of light source units are used in combination, since light emitted from a single light source unit is not sufficient to provide a required amount of light; and in addition, irradiation ranges of such light source units are set to be different so that by way combining them an upper beam pattern (a high beam distribution pattern) and a lower beam pattern (a low-beam distribution pattern) are created.
For example, U.S. Pat. No. 6,882,110 discloses a technology for providing a low-beam distribution pattern by combining a cut-off line forming light source unit, a hot zone forming light source unit, and a diffusion region forming light source unit, and it further discloses a technology for adjusting to various low-beam distribution patterns by changing combinations of the light source units. Japanese Patent Application Laid-Open (Kokai) No. 2005-324657 discloses a technology for controlling current supplied to some of LEDs installed in a headlamp depending on vehicle speed and separately controlling current supplied to the other LEDs, so that the light distribution characteristics such as irradiation ranges and directions can be adjusted. Further, United States Patent Application No. 2005180139 discloses a technology for providing different light distribution characteristics by varying the ratios of the amount of light emitted by each one of the LEDs.
According to U.S. Pat. No. 6,882,110, since the light distribution patterns are uniformly determined by combinations of the plurality of light source units installed in the headlamp, it is difficult to change the light distribution patterns in response to the changes in driving status of the vehicle.
On the other hand, in Japanese Patent Application Laid-Open (Kokai) No. 2005-324657 and United States Patent Application No. 2005180139, electric current supplied to the LEDs is controlled so as to change the brightness of the LEDs and thus to change the light distribution patterns. In these conventional lamps, the light distribution patterns are changed in response to the changes of the driving status of the vehicle. However, in these Japanese related art, since the current supplied to some of the LEDs is increased or decreased, the amount of light emitted from the LEDs increases or decreases accordingly, resulting in that the total amount of light emitted from the headlamp a becomes higher or lower. This means that the brightness in the vehicle's front area illuminated by the headlamp varies depending on the changing light distribution patterns, making the driver sometimes feel discomfort. The driver even feels unease particularly when the brightness in the vehicle's front area is lowered. As a result, Japanese Patent Application Laid-Open (Kokai) No. 2005-324657 and United States Patent Application No. 2005180139 in fact increase the amount of light emitted from the LEDs but does not decrease the amount of light emitted from the LEDs in order to secure the safety for driving conditions. However, when the amount of light from the LEDs increases, the power consumption of the headlamp increases, resulting in that the burden on the vehicle's battery becomes higher.